The packaging industry has long sought to develop plastic film, sheet, bottles, wrappings, and containers which are impervious to oxygen for preserving oxidizable materials and oxygen-sensitive foods and beverages. That industry has further sought to develop similar materials resistant to the passage of carbon dioxide for use in maintaining the carbonation of carbonated beverages. Resistance to passage of water vapor is also important to the packaging industry. No organic polymeric materials are truly impervious to gases; they all show some degree of permeability. Those which have low permeability to a particular gas are considered to be good barriers for that gas, while those which have high permeability are considered to be non-barrier materials with respect to that particular gas. The most useful polymers which exhibit very low values for oxygen permeability, i.e., are good barriers to oxygen, are poly(vinylidene chloride) and polymers containing vinyl alcohol mers, such as ethylene-vinyl alcohol copolymers containing less than about 50 mol percent ethylene units, or the homopolymers of hydrolyzed poly(vinyl acetate) known as poly(vinyl alcohol)s.
Although each of these types of polymers is utilized in commerce, they have deficiencies which limit their broader use. Poly(vinylidene chloride) is thermally less stable than most polymers and is difficult to process; poly(vinyl alcohol)s are difficult to process, and their barrier properties are greatly affected by high relative humidity, and the ethylene-vinyl alcohol polymers, which are more easily processed than poly(vinyl alcohols not containing ethylene, are also sensitive to moisture, and are not optically clear. Further, the structural properties required for many applications are difficult to achieve with these polymers.
The packaging industry has also sought to prepare containers exhibiting enhanced service temperature for the hot-fill packaging of foods, sterilization prior to packaging, autoclaving to sterilize contents, and the like. Materials attractive for such heat-sensitive uses tend to have poor barrier properties.
Polymers based on lower alkyl methacrylates, such as poly(methyl methacrylate) exhibit clarity, have some degree of toughness, and can be compounded with impact modifiers to improve toughness, but do not exhibit satisfactory barrier properties. Conversion by reaction with lower alkyl mines or ammonia to polymers with mers of glutarimide improves the barrier properties significantly, but they still do not meet the requirements of the most demanding barrier applications. Kopchik, U.S. Pat. No. 4,246,374, discloses such polymers in thermally stable form, and discloses that their barrier properties are superior to most clear thermoplastics.
Hallden-Abberton et al., in U.S. Pat. No. 4,727,117, describe a means of reducing the content of unreacted acid and anhydride groups of glutarimide polymers to prepare novel polymers of even higher thermal stability. In that patent, an extensive list of polymers with which such acid-content reduced glutarimides may be blended is given. Among these polymers are listed ethylene-vinyl acetate polymers and polyvinyl alcohol. Ethylene-vinyl alcohol polymers have not been disclosed for this use, and there is no suggestion in the prior art that the permeability behavior of such blends would differ in any way from an expected average performance, nor that the resulting blend would be particularly useful in barrier packaging applications. There is further no suggestion that blends of a second polymer with the acid-reduced polyglutarimides could be admixed with the polymers containing vinyl alcohol mers to obtain the barrier properties of the present invention.
Blends of ethylene-vinyl alcohol polymers with vinyl alcohol contents greater than about 50 mol percent have been noted in the patent literature as components of blends with certain matrix polymers, such blends having attractive barrier properties. Particularly noted as matrices are poly(vinyl chloride) in U.S. Pat. No. 4,003,963, poly(ethylene terephthalate) in US Pat. No. 4,284,671, and polypropylene in U.S. Pat. No. 4,362,844. These patents do not suggest the use of ethylene-vinyl alcohol copolymers with acrylic or glutarimide polymers for such purposes, nor do they disclose unexpected improvement in barrier properties at low levels of the ethylene-vinyl alcohol copolymer in the blend.
Particularly noted is the intensive study of polyamide blends with ethylene-vinyl alcohol resins. U.S. Pat. No. 4,427,825 teaches such blends, wherein there are regions of the ethylene-vinyl alcohol copolymer having an average diameter of less than 50 nanometers in the polyamide. These compositions, as do those of the blends noted in the previous paragraph, exhibit a linear relationship for permeability behavior which is expected for such blends and which is demonstrated by a straight-line plot when the permeability is plotted as the ordinate on semi-logarithmic paper with ethylene-vinyl alcohol content as the abscissa.
One known exception to this additive relationship of the linear logarithm of permeability is the disclosure in U.S. Pat. No. 4,410,482 of blends of a polyolefin matrix and a modified or dispersed nylon polymer, in which the nylon is processed to form laminar domains in the matrix. Such blends exhibit barrier properties against hydrocarbon liquids and gases substantially improved over that predicted for a homogeneous blend. Processes are also taught for the incorporation into the polyolefin matrix of dispersed similar platelets of other polymers, such as polycarbonate and poly(butylene terephthalate). This patent does not teach or suggest the utility of ethylene-vinyl alcohol to form laminar structures in poly(glutarimides); indeed, it requires a solvent or a dispersant polymer to obtain the laminar structures, whereas the present invention achieves the desired structure by a conventional thermal/mechanical history of mixing. This patent also does not teach or suggest that the combination of ethylene-vinyl alcohol or poly(vinyl alcohol) polymers within a polyglutarimide matrix will produce unexpectedly good resistance to the passage of gases- Further, the platelet or laminar morphology shown by this patent does not necessarily correspond with the morphology of the present blends; in some examples of improved barrier properties exemplified herein, the vinyl alcohol (co)polymer is finely dispersed in very small particles uniformly throughout the glutarimide matrix, and in others, the vinyl alcohol (co)polymer blend with the glutarimide or poly(lower alkyl) methacrylate matrix polymer shows optical clarity, a single glass-transition temperature, and/or other indicators of blend compatibility. Certain blends at relatively high levels of certain ethylene-vinyl alcohol copolymers do produce a laminar structure with excellent barrier performance.
European Patent Application No. 273,897 discloses blends of polyethylene terephthalate with styrene-maleic anhydride as blow-molded containers having an oriented crystalline continuous phase of polyethylene terephthalate containing dispersed ovoid particles of the styrene copolymer, the ovoids having a diameter of 0.1 to 0.8 .mu.m and a length of 0.3 to 2 .mu.m. The microstructure is reported to impart physical properties suitable for containment or protection against permeation of gases or organic fuels.
A publication in Research Disclosure, October, 1988, page 726, discusses the barrier properties of blends of poly(ethylene naphthalene carboxylate) with relatively low levels of ethylene-vinyl alcohol copolymers. This publication states that the actual values for oxygen permeability in such blends are four to five times lower than predicted from calculations of the effective permeability of the ethylene-vinyl alcohol copolymer from PET data. These data, while not calculated against a pure ethylene-vinyl alcohol polymer standard and thus not directly comparable with the data of the present invention, do show an unexpected improvement in a manner also demonstrated herein. The composite materials of the reference are opaque; the domains which are necessary to lower the oxygen permeability must be large enough to eliminate the clarity of the blends and thus destroy one of the particularly useful properties of the matrix polymers.
It is thus an object of the present invention to prepare a blend of polymers having outstanding barrier properties to oxygen, carbon dioxide and moisture from one or more polymers having certain desirable physical properties but inadequate gas barrier properties of their own, and a polymer with vinyl alcohol mers which has excellent barrier properties. Another object is to prepare such a barrier blend structure further having excellent optical properties, resistance to impact, a service temperature sufficient for hot-fill and sterilization, and/or other desirable physical properties. Further objects and advantages will be apparent from the following description of the present invention.